1. Field of the Invention
The present invention relates to an integral transaxle apparatus employing a hydrostatic and mechanical axle driving and steering system, including two hydrostatic transmissions (hereinafter, each hydrostatic transmission is called an "HST") of the same type; one for driving left and right axles, and the other for steering the axles.
2. Related Art
A well-known conventional system for driving and steering left and right axles for wheels or sprockets of tracks, which employs a pair of HSTs provided for the respective axles, is disclosed in U.S. Pat. No. 4,782,650 or Japanese Laid Open Gazette No. Hei 2-261,952, for example. Output rotary speeds of the two HSTs are equalized for straight driving of a vehicle and made different from each other for steering of it.
The conventional system has the problem that the equalization of output rotary speeds between the two HSTs must be precise and if there is difference of capacity between the two, a vehicle is steered differently between leftward and rightward cornering.
The present invention includes an axle driving and steering system including two HSTs and a differential unit. In this regard, one of the HSTs (a driving HST) interlocks with a speed changing operation tool such as a lever or a pedal and transmits power from an engine into the differential unit so as to drive left and right axles forwardly or reversely. The other HST (a steering HST) interlocks with a steering operation tool such as a steering wheel and transmits its output rotational force corresponding to the direction and degree of the steering operation tool into the differential unit so as to differentially drive the axles.
However, even in such a system, the problem still remains that the individual elements serving as the two HSTs and the differential unit, and power transmission mechanisms interlocking the hydraulic motors of the two HSTs with the differential unit are so complicated as to hinder the system from being compacted as an integral apparatus.
Furthermore, the two HSTs should be fed with efficiently cooled fluid while a charge pump therefor is preferably compact.